How does the practice feature work?

There's that big button that says practice, but it makes me practice stuff from the first lesson of a language. Why can't I select a certain skill, or certain skills, to practice? I don't need to review really basic stuff, I want to review stuff from more recent lessons. Please help me.

Without going into many details (just a mostly relevant overview), a few years ago the practice button (then the strengthen button) was more likely to give you material from recent skills.

Duolingo changed that, and made it so it gave you material in the same order as the course, from skills it thinks haven't been seen in a while. (It may be more sophisticated than that, but it doesn't seem it. In fact I have seen it seem to skip skills which were at level 5 while selecting material, but that could have just been coincidence, and I have seen no official information that it does this.)

Once Duolingo crowns were introduced one year ago, the way to repeat recent material (at least without using third party websites) became first to work up through the crown levels, then once a skill reached level 5 to click on it again and click "practice" on the specific skill dropdown.

TL;DR;

Which is to say the helpful practice button you see near the top of the course does not do what you want, and you should revisit the specific skill instead.